1. Field of the Invention
This invention relates to a method of forming a hermetic seal, and more particularly, to a method of making a hermetic seal for electronic image sensor packages.
2. Background of the Prior Art
In the packaging of solid-state devices, a semiconductor chip is typically mounted within a cavity formed in a ceramic housing, and a cover is sealed to the housing to close the cavity. In U.S. Pat. No. 4,750,665, there is disclosed a method of producing a semiconductor package in which a ceramic cover is joined to a ceramic housing using a gold-tin solder paste. When the solder has been applied and the cover and housing have been assembled together, they are heated to a temperature of about 350.degree. C. to form a bond between the parts. There is a problem in using such a process in the packaging of semiconductor devices which contain some types of image sensors. Certain image sensors include color filter arrays and lenslets which are formed of organic materials, and the temperatures used to melt the solder in the patented process can cause degradation of the organic materials.
Epoxy materials have been used to seal the packages of solid-state devices containing image sensors in order to avoid the high temperatures used in sealing semiconductor packages with solder. The epoxys, however, do not form a hermetic seal, and thus, they are not suitable for some applications. For example, solid-state devices used in military applications must meet the requirements set forth in a military specification, Mil-Std-833C, in which the packages must be able to withstand pressurized tests.
It is known in the prior art to bond crystals together using low-melting-point alloys and pressure fusing under relatively low temperatures. In U.S. Pat. No. 4,077,558, to Carlson et al., there is disclosed a method of bonding two crystal components together to form an acousto-optic device. A bond enhancing material, such as Cr, is coated on one crystal, and a layer of noble metal, such as gold, is then deposited on the bond enhancing material. On the other crystal, a low-melting-point metal, such as tin or indium, is coated on the crystal, and then a noble metal is coated over the low-melting-point metal. The two noble metal layers are then brought together at a temperature of 100.degree.-200.degree. C. and at a pressure of at least 50 psi. The low-melting-point material will diffuse into the noble metals and across the interface, causing a bond to form by elimination of the boundary between the crystals. Although the highest temperatures used in this process are somewhat lower than in the patent discussed above, the temperatures are still too high to prevent damage to the image sensors. The most serious drawback, however, to the process disclosed in the Carlson et al. patent is the very high pressures which must be applied to cause the diffusion of the noble metal and the low melting point alloy. Such high pressures could not be used to seal semiconductor packages.